9 Distributed Applications

In a distributed system with several Erlang nodes, there may be
a need to control applications in a distributed manner. If
the node, where a certain application is running, goes down,
the application should be restarted at another node.

Such an application is called a distributed application.
Note that it is the control of the application which is
distributed, all applications can of course be distributed in
the sense that they, for example, use services on other nodes.

Because a distributed application may move between nodes, some
addressing mechanism is required to ensure that it can be
addressed by other applications, regardless on which node it
currently executes. This issue is not addressed here, but the
Kernel module global or STDLIB module pg can be
used for this purpose.

Distributed applications are controlled by both the application
controller and a distributed application controller process,
dist_ac. Both these processes are part of the kernel
application. Therefore, distributed applications are specified by
configuring the kernel application, using the following
configuration parameter (see also kernel(6)):

distributed = [{Application, [Timeout,] NodeDesc}]

Specifies where the application Application = atom()
may execute. NodeDesc = [Node | {Node,...,Node}] is
a list of node names in priority order. The order between
nodes in a tuple is undefined.

Timeout = integer() specifies how many milliseconds to
wait before restarting the application at another node.
Defaults to 0.

For distribution of application control to work properly,
the nodes where a distributed application may run must contact
each other and negotiate where to start the application. This is
done using the following kernel configuration parameters:

sync_nodes_mandatory = [Node]

Specifies which other nodes must be started (within
the timeout specified by sync_nodes_timeout.

sync_nodes_optional = [Node]

Specifies which other nodes can be started (within
the timeout specified by sync_nodes_timeout.

sync_nodes_timeout = integer() | infinity

Specifies how many milliseconds to wait for the other nodes
to start.

When started, the node will wait for all nodes specified by
sync_nodes_mandatory and sync_nodes_optional to
come up. When all nodes have come up, or when all mandatory nodes
have come up and the time specified by sync_nodes_timeout
has elapsed, all applications will be started. If not all
mandatory nodes have come up, the node will terminate.

Example: An application myapp should run at the node
cp1@cave. If this node goes down, myapp should
be restarted at cp2@cave or cp3@cave. A system
configuration file cp1.config for cp1@cave could
look like:

When all involved (mandatory) nodes have been started,
the distributed application can be started by calling
application:start(Application) at all of these nodes.

It is of course also possible to use a boot script (see
Releases) which
automatically starts the application.

The application will be started at the first node, specified
by the distributed configuration parameter, which is up
and running. The application is started as usual. That is, an
application master is created and calls the application callback
function:

Module:start(normal, StartArgs)

Example: Continuing the example from the previous section,
the three nodes are started, specifying the system configuration
file:

If the node where the application is running goes down,
the application is restarted (after the specified timeout) at
the first node, specified by the distributed configuration
parameter, which is up and running. This is called a
failover.

The application is started the normal way at the new node,
that is, by the application master calling:

Module:start(normal, StartArgs)

Exception: If the application has the start_phases key
defined (see Included Applications), then the application is instead started
by calling:

Module:start({failover, Node}, StartArgs)

where Node is the terminated node.

Example: If cp1 goes down, the system checks which one of
the other nodes, cp2 or cp3, has the least number of
running applications, but waits for 5 seconds for cp1 to
restart. If cp1 does not restart and cp2 runs fewer
applications than cp3, then myapp is restarted on
cp2.

Figure
9.2:
Application myapp - Situation 2

Suppose now that cp2 goes down as well and does not
restart within 5 seconds. myapp is now restarted on
cp3.

If a node is started, which has higher priority according
to distributed, than the node where a distributed
application is currently running, the application will be
restarted at the new node and stopped at the old node. This is
called a takeover.

The application is started by the application master calling:

Module:start({takeover, Node}, StartArgs)

where Node is the old node.

Example: If myapp is running at cp3, and if
cp2 now restarts, it will not restart myapp,
because the order between nodes cp2 and cp3 is
undefined.

Figure
9.4:
Application myapp - Situation 4

However, if cp1 restarts as well, the function
application:takeover/2 moves myapp to cp1,
because cp1 has a higher priority than cp3 for this
application. In this case,
Module:start({takeover, cp3@cave}, StartArgs) is executed
at cp1 to start the application.